(a) Field of the Invention
The present invention is directed in general to an audio distortion measurement system and more particularly to a system which includes an ultra low distortion oscillator which may also provide for improved amplitude control, includes a distortion analyzer which has an improved notch filter system, and includes a combination of such analyzer and oscillator forming the overall system where all system components effectively track together and provide for fast stabilization.
(b) Description of Prior Art
In audio distortion measurement systems such as shown, for example, in U.S. Pat. Nos. 3,890,570 and 3,978,401, which may find use in measuring the distortion of audio amplifiers, for example, it is desired to measure distortion to below -95 dB. Moreover, it is desired to make several measurements using various frequencies and signal levels to fully qualify the device under test. The system must provide convenient and rapid means for changing these parameters and rapidly yielding a measured result. In some applications, automatic selection of a sequence of frequency and signal level parameters must be made using an external computer or other control.
Furthermore, the ability of such a system to measure distortion quantities to below -95 dB requires that the oscillator generating the test signal and the analyzer measuring the received test signal have inherent or contributed distortion substantially below this figure.
Prior art has produced systems meeting this performance capability but not the rapid manual parameter selection or external programmable parameter selection. Conversely, prior art exhibits programmable systems yielding computer control capability but not meeting the performance criterion.
A typical prior art distortion analyzer is produced by Sound Technology Inc., Campbell, Calif., as model 1700B or 1701A. Various features of this system are shown in the above-mentioned U.S. Pat. No. 3,978,401 in the name of Jack G. S. Lum assigned to Sound Technology, Inc. In general, all distortion analyzers known to the Applicants operate on the principle of selectively rejecting a fundamental frequency component and then measuring the remaining components which, of course, make up the distortion of the main waveform.
In general, a classic problem in the design of a low distortion oscillator and distortion analyzer is that both components of the distortion measurement system must be varied over a wide frequency range and signal level range in order to perform the necessary function of the system in measuring the distortion of signals of various frequencies and levels, for example, over the audio range. To change frequencies requires the changing of resistive-capacitive components. This introduces component variations because of the tolerances and mistracking of such components. In order to compensate for such variations feedback adjustments are necessary. However, such feedback adjustments in turn introduce distortion thus defeating the entire purpose of providing a distortion measurement system.
Prior devices have attempted to meet the foregoing problems but have only partially done so. Most important no prior system can effectively measure in a simple and economical manner the low distortion which are now included in the specifications of modern receiver-amplifiers. No prior system lends itself to external parameter control or rapid manual control.